High-temperature Stability Of Precursor Conversion T-BN_W And Properties Of SiO₂ Ceramic Reinforced By T-BN_W

Large aspect ratio（>50）boron nitride（BN）reinforcements with excellent high-temperature stabilities have become the pressing needs in the wave transparent material.However,the existing techniques can hardly prepare high quality BN reinforcements with large aspect ratio on a large scale,which restricts the key technology and application of boron nitride reinforced fused silica（BN/Si O₂）composite ceramics.In this regard,the present work intended to controllably prepare the turbostratic boron nitride wire（t-BN_W）with large aspect ratio by combining self-formed melamine borate（C₃N₆H₆（H₃BO₃）₂）precursors with a three-step pyrolysis process.We focused on the the pyrolysis mechanisms and behaviour of C₃N₆H₆（H₃BO₃）₂,as well as the high-temperature stability of the novel t-BN_W.In addition,the microstructural evolution,toughening mechanisms and thermal shock damage behaviour of BN_W/Si O₂ composite ceramics were reavled by the assistance of SEM,TEM and mechanical testing machine.Melamine（C₃N₆H₆）and boric acid（H₃BO₃）form intermolecular hydrogen bondings to build C₃N₆H₆（H₃BO₃）₂.BN_W with smooth surface and low carbon residual can not be obtained by the one-step pyrolysis process（1600℃/N₂/3 h or1200℃/NH₃/3 h）.The smooth t-BN_W can be obtained using a three-step pyrolysis（500℃/air/3 h,800℃/N₂/1 h,1100℃/NH₃/3 h）,the carbon residue is low as 0.25wt.%and average diameter is 2.28μm.In RT-500℃（air）,the dehydration of H₃BO₃domain in C₃N₆H₆（H₃BO₃）₂,evaporation of C₃N₆H₆,condensation of C₃N₆H₆,crosslinking reaction between BO oligomer and C₃N₆H₆,and the breaking of triazine ring sequentially occur,forming the amorphous BNOCH and traceable B₂O₃.The reactions of-B-O-with its own-C=NH,-B-N=C-occur in RT-800℃（N₂）,realizing the transformation of B-O₃→O₂-B-N→O-B-N₂→B₃-N₃.NH₃further nitriding amorphous BNO and traceable B₂O₃ to form amorphous BN（a-BN）in the high-temperature nitriding stage,and the structure of a-BN is rearranged to form t-BN under high temperature.t-BN_W exhibits an initial oxidation temperature（～950℃）comparable to that of h-BN_P（D50=10.4μm）and excellent oxidation resistance.The oxidation resistance of t-BN_W is higher than those of the commercial carbon fiber（C_f）and Hi-Nicalon silicon carbide fibre（Si C_f）,ascribed to the B-N covalent bond which is stronger than those of C_f and Hi-Nicalon Si C_f.The relatively high disorder and tiny amounts of easily oxidized impurities（C and H）lead to the lower oxidation activation energy of t-BN_W（202.9 k J/mol）compared to h-BN_P（330 k J/mol）with a diameter of 10.4μm.The reactive gas adsorption in surface and active sites formation induce the oxidation induction period,the duration time is mainly determined by the surface defect density.The oxidation behaviour of t-BN_W is determined by the internal microstructure and surface morphology,controlled by the surface state and geometric morphology,and presents an isotropic homogeneous oxidation behaviour.According to the model-fitting method,with the increasing oxidation degree（α）,the oxidation behaviour of t-BN_W can be divided into three stages:two-dimensional random nucleation（0-5%）,two-dimensional phase boundary reaction（5%-70%）and three-dimensional diffusion control（70%-75%）.The average oxidation activation energy is 234.03 k J/mol in the two-dimensional random nucleation;which is controlled by the diffusion of oxygen through the B₂O₃melt（168.50 k J/mol）in the three-dimensional diffusion.Additionally,t-BN_W shows a better thermal stability at high temperature in Ar compared with N₂,after teatment at 1600℃/Ar/1 h,the turbostratic structure is almost unchanged,and the relative degree of crystallization is only 15.56%of that in N₂.The densification of BN_W/Si O₂ composite ceramics is basically realized at1300℃/30 MPa/10 min.The microstructure is controlled by sintering temperature,Si O₂ matrix undergos crystal nucleus breeding（≤1250℃）,followed by grain nucleation（1250-1350℃）,and then grain growth（1350℃）;The t-BN_W presents uniform stable structure（1250℃）and surface differed core contrast structure（1350℃）and skin-core structure（1450℃）.The high-frequency abrupt change in the crack propagation direction results in the increase of fracture surface energy,the fracture toughness of 20 wt%BN_W/Si O₂ composite ceramic is 3.4 times higher than that of bulk fused silica.Additionally,the hardness is reduced to 2.8 GPa due to the soft phase of t-BN_W,consequently,the machinability parameters（P^*）fold increases,showing an excellent machinability.The bending strength of 20 wt.%BN_W/Si O₂composite ceramic decreases as the temperature elevate（RT-1000℃）,and then increases slowly after 800℃,which is an equilibrium result of the strength degradation of t-BN_W,blunting of crack tip,stress relaxation and surface defects healing.BN_W/Si O₂ composite ceramics have good dielectric properties,the porosity controlled by sintering temperature has an absolute influence on the dielectric properties.t-BN_W has little effect on the dielectric constant;when porosity is considerable,high content of t-BN_W leading to more BN_W-Si O₂ interface scattering and diffuse pore interface scattering will significantly increase the dielectric loss.The BN_W/Si O₂ composite ceramics have stable and small thermal expansion coefficients,attributed to the turbostratic structure and the lack of considerable cristobalite.The 20 wt.%BN_W/Si O₂ composite ceramic exhibits an excellent thermal shock resistance（?T=0-1200℃）,and the residual strength ratio are all higher than 70%.Based on the thermal-shock fracture theory and the thermal-shock damage theory,the thermal shock evaluation model including the temperature-dependent properties of materials can reveal the mechanism of thermal shock damage reasonably.A three-step pyrolysis method of C₃N₆H₆（H₃BO₃）₂ whiskers is proposed to prepare large aspect ratio t-BN_W for the first time,this work elucidates the high-temperature stability in inert atmosphere and antioxidation mechanism in oxidation atmosphere of the novel t-BN_W,also reveales the effect of t-BN_W on the densification,microstructure,mechaninical and dielectric properties of BN_W/Si O₂composite ceramic.This work extend the development of BN reinforcements,also enriches the theoretical insight and experimental date about BN/Si O₂ system ceramics.